Pressure cookers don’t boil enough to emulsify the fat and incorporate it into the broth. Kenji tried it and found it lacking
What emerged from the pressure cooker sure was tasty, but it was by no means a good tonkotsu ramen broth. Rich with gelatin and flavor, to be sure, but it was nearly transparent in color. The problem is that in the high-pressure environment of a pressure cooker, temperatures get higher, allowing for fast extraction and conversion of collagen to gelatin, but the high pressure also prevents the rolling boil necessary for getting those extracted solids to emulsify into the broth.
To be clear, I’ve made almost every kind of ramen, at this point it’s mostly about perfecting a single style that both myself and my partner enjoy (she doesn’t really like tonkotsu due to it’s richness)